ISLAMABAD — A fiery anti-government cleric on Saturday led a massive rally of thousands of protesters in Pakistan's capital who defied pouring rain to demand the prime minister step down over alleged fraud in last year's election.

Cleric Tahir-ul-Qadri, along with famous cricketer-turned politician Imran Khan, have drawn tens of thousands of demonstrators to dual protests that have disrupted life across Islamabad. They demand Prime Minister Nawaz Sharif step down over alleged fraud in the country's May 2013 election, something Sharif has refused to do.

The protesters have vowed to remain in the streets until Sharif leaves office, raising fears of political instability in the nuclear-armed nation, which only saw its first democratic transfer of power last year.

On Saturday, Qadri told his supporters to continue protesting until they bring about a "peaceful revolution."

"Nawaz Sharif should be arrested when he steps down and he should not be allowed to leave the country," he said. He also called for the dissolution of Parliament and fresh elections.

Sharif has given no indication he intends to step down, and leaders of his party said Qadri's demands were unconstitutional.

Khan, who has camped with his supporters less than one kilometer away from Qadri's rally, also vowed Saturday to continue his protest.

"I will not go back to my home without getting resignation from Nawaz Sharif," he told reporters.

Both Khan and Qadri have vowed to bring 1 million followers into the streets of Islamabad, a city of roughly 1.7 million inhabitants. Police estimated that nearly 35,000 people were present at Qadri's rally and 25,000 at Khan's.

Shortly after Qadri's speech, senior Cabinet minister Ahsan Iqbal urged the two opposition leaders to step back from their demands that Sharif step down.

"We do hope that Imran Khan and Tahir-ul-Qadri will show flexibility" to end the political instability, Iqbal said.

Qadri, who is also a Canadian national, commands a loyal following of thousands through his network of mosques and religious schools in Pakistan. Last year, Qadri held a protest in the capital calling for vaguely worded election reforms ahead of the country's May poll, grinding life in Islamabad to a halt.

Khan meanwhile helms the Pakistan Tehreek-e-Insaf party, which is the third-largest political bloc in parliament.

Ahead of the protests, security forces put shipping containers on streets as roadblocks and stationed riot police around the capital.

Pakistan, a nuclear-armed country of 180 million people, has largely been ruled by military dictators since it was carved out of India in 1947. The army still wields great influence in Pakistan, which is battling several militant groups, but has not taken sides in the protests. There are fears, however, that political unrest could prompt the military to intervene.

Sharif was himself overthrown in the 1999 coup that brought former army chief Pervez Musharraf to power.

Pakistan's military is currently fighting militants in the country's North Waziristan tribal region in an effort to eliminate extremists accused of orchestrating attacks in Pakistan and neighboring Afghanistan.